Buying un-registered property and seller asking us to clear loan before registration

I am buying an apartment in Bangalore from X and Y (husband and wife) X&Y had purchased this flat from a reputed builder, and they have also taken possession. X and Y have not yet registered the flat. (the bank loan is taken by X and it is not a joint loan) Please note that Y is also an employee of the same reputed builder. We have made a triparty assignment agreement, where X and Y (Seller) are first-party, I am (Buyer) second part Builder as the third party We have made a triparty assignment agreement and paid the 20% of the amount with 2 cheques one favouring X and one favouring Y (husband and wife, since both their name, are in the assignment agreement also) I and my wife have now taken a joint bank loan for the remaining amount the same is approved and the bank is ready to disburse the loan amount on the day of registration. (Note: I have taken a fresh loan from the same bank as X&Y) Problem: Now the seller (X and Y) is asking us to tell the bank to transfer and clear their loan first so they save paying interest to the bank. He(Y) says that he has spoken to the banker and they will do it if we request them to do so. He(Y) is saying that since we have signed an assignment agreement, the builder will change their name to ours and that they are not required from now on, and also not required to come on the day of registration as the builder will take care of everything. They have cleared the pending payments to the apartment association and we have a NOC, but we haven't received any NOC or any communication from from the builder. We are telling them both to come on the day of registration and sign as consenting parties and take the cheque once registration is done, to which they are not agreeing. He(Y) is saying that his wife(X) is not in town and cannot come and he cannot wait till the registration happens as he will lose a lot of money paying the bank interest. He (Y) is also saying that this is how they do usually and there is no problem at all. He is not keen to come for registration and he is saying that he will give a GPA to my father so he can sign on their behalf. My question is: Should we wait till the registration just to be safe or ask the bank to disburse the loan and later do the registration? Is it mandatory for them both to come for registration or any one of them can come? What are the documents we will need from them at the time of registration? Can he(Y) give a GPA to anyone to sign on their behalf? Please advise on how to go about this?